The present invention relates generally to analog signal isolation circuits. More particularly, the present invention relates to control signal interface circuits for providing galvanic isolation and protection against line voltage application across analog control signal input terminals, such as for use with electronic ballasts.
Generally speaking, a control signal interface circuit is designed for connection to an external source such as a DC control voltage, while an associated electronic ballast is designed for connection to a mains AC power source. A conventional control signal interface circuit may be designed to deliver a small current, with a DC control voltage obtained from a variable resistor, shunt regulator, light-level controller, etc. Shunt regulators for example may control electronic ballasts by setting the voltage across the control input terminals to a predetermined voltage range, wherein upon disconnection of the shunt regulator from the control interface the open-circuit voltage across the control terminals rises to approximately the upper end of the range.
In designing for these connections, electronic ballasts and associated interface circuitry usually include a pair of control signal input terminals for connection to the DC control voltage and a pair of mains AC power input terminals for connection to the external mains power source. However, an all-too common problem during the installation of electronic ballasts is where control signal input terminals of the ballast are inadvertently coupled to a mains line input source. Such an error may result in a great deal of damage to interface circuitry within the electronic ballast. Further, where a single interface circuit may be provided for a plurality of electronic ballasts, such damage may extend to an array of ballasts and multiply the costs of replacement accordingly.
Protection circuits have previously been provided and are known in the art for protecting the control interface circuitry in electronic ballasts against line voltages. In response to the application of line voltages, high impedance is provided to limit current in the protection circuit and clamping circuitry may be further provided to limit the output voltage from the protection circuit to the interface circuitry and the remainder of the electronic ballast generally. However, such circuits typically also utilize PTC thermistors or high voltage transistors to provide such protection, which increases the cost of the circuit.
It would be desirable to provide a relatively low cost interface circuit with sufficient protection against the application of line voltages.